Commercial and recreational freshwater fisheries are important to the economy
of many regions, as well as the well-being of native populations. In many aquatic
ecosystems, freshwater fish also are important in maintaining a balance in other
aquatic populations lower in the food web (via predatory and other effects).
In broader terms, aquatic ecosystems are important as recreational areas, as
sources of water for domestic and industrial use, and as habitat for a rich
assemblage of species, including some that are threatened or endangered.

Several studies have indicated that projected climate change will have important
impacts on North American freshwater fisheries and aquatic ecosystems. It must
be noted, however, that most studies to date have used results from earlier
climate model simulations that gave air temperature increases under a 2xCO2
climate that were as much as twice as large for the same time period as more
recent estimates that include aerosol forcing-thus overestimating the effects
of temperature increases, particularly in the summer.

Changes in survival, reproductive capacity, and growth of freshwater fish
and the organisms and habitats on which they depend result from changes in water
temperature, mixing regimes, and water quality.

In North America, freshwater fish have been grouped into three broad thermal
groups (cold-water, cool-water, and warm-water guilds) based on differences
in the temperature optima of physiological and behavioral processes. In simulations
of deep, thermally stratified lakes in the mid- and high latitudes, including
the Laurentian Great Lakes, winter survival, growth rates, and thermal habitat
generally increase for fish in all three thermal guilds under the 2xCO2 climate
(DeStasio et al., 1996; IPCC 1996, WG II, Sections 10.6.1.2 and 10.6.3.2; Magnuson
and DeStasio, 1996). However, in smaller mid-latitude lakes, particularly those
that do not stratify or are more eutrophic, warming may reduce habitat for many
cool-water and cold-water fish because deep-water thermal refuges are not present
or become unavailable as a consequence of declines in dissolved oxygen concentrations
(IPCC 1996, WG II, Section 10.5.4). For example, Stefan et al. (1996) examined
the effect of temperature and dissolved oxygen changes in lakes in Minnesota;
they projected that under a 2xCO2 climate (from a GISS GCM that projected a
3.8°C air temperature increase in northern Minnesota), cold-water fish species
would be eliminated from lakes in southern Minnesota, and cold-water habitat
would decline by 40% in lakes in northern Minnesota.

Changes in the productivity and species composition of food resources also
may accompany climatic warming and, in turn, influence fish productivity. Production
rates of plankton and benthic invertebrates increase logarithmically with temperature;
rates increase generally by a factor of 2-4 with each 10°C increase in water
temperature, up to 30°C or more for many organisms (Regier et al., 1990; Benke,
1993; IPCC 1996, WG II, Section 10.6.1.1). Although this effect generally should
increase fish productivity, shifts in species composition of fish prey with
warming might prevent or reduce productivity gains. Biogeographic distributions
of aquatic insects are centered around species thermal optima, and climate warming
may alter species composition by shifting these thermal optima northward by
about 160 km per 1°C increase in temperature (Sweeney et al., 1992; IPCC 1996,
WG II, Section 10.6.3.1). If species range shifts lag changes in thermal regimes
because of poor dispersal abilities or a lack of north-south migration routes
(e.g., rivers draining northward or southward) or if species adaptation is hindered
by limited genetic variability, climatic warming might result initially in reductions
in the preferred prey organisms of some fish (IPCC 1996, WG II, Section 10.6.3.3).

Climatic warming may result in substantial changes in the thermal regimes and
mixing properties of many mid- and high-latitude lakes. In the mid-latitudes,
some lakes that presently are dimictic (mixing in spring and autumn) may no
longer develop winter ice cover and may become monomictic (mixing during fall,
winter, and spring), with a longer summer stratification period. At high latitudes,
some lakes that presently are monomictic and mix during summer may stratify
in summer and mix twice a year, in autumn and spring (IPCC 1996, WG II, Section
10.5.4). Changes in lake mixing properties may have large effects on hypolimnetic
dissolved oxygen concentrations (affecting available fish habitat) and on epilimnetic
primary productivity, although these effects are likely to depend greatly on
the morphometric characteristics of individual lakes and are difficult to predict
(IPCC 1996, WG II, Section 10.5.4). For example, longer summer stratification
and higher water temperature result in more severe hypolimnetic oxygen depletion
in lakes in Minnesota under a 2xCO2 climate simulation (Stefan et al., 1993).
In other lakes, reduction in the duration or lack of winter ice cover might
reduce the likelihood of winter anoxia (IPCC 1996, WG II, Section 10.6.1.4).
At high latitudes, development of summer stratification under a warmer climate
might increase lake primary productivity by maintaining algae for longer periods
within the euphotic zone. Climate changes that result in decline in runoff also
may have substantial effects on the mixing properties of smaller lakes that
are heavily influenced by fluxes of chemicals from their catchments. For example,
the surface mixed layer of boreal lakes at the Experimental Lakes Area in northWest
Ontario has deepened over the past 20 years as a result of a long-term drought
that reduced inputs of DOC from the catchment and thus increased water clarity
(IPCC 1996, WG II, Section 10.5.3 and Box 10-2; Schindler et al., 1996).